Sendly vs Customer.io
Customer.io is multi-channel messaging driven by events. Sendly does the email channel of that job — events, segments, branching workflows — and nothing else, on a per-email price.
Last updated 2026-09-04
The short version
Customer.io is an event-driven messaging platform. You stream behaviour into it, build campaigns that branch on that behaviour, and reach people across several channels.
Sendly shares the mechanism and not the breadth. You can track custom events against a contact, build dynamic segments from contact data and behaviour, and run visual workflows that branch on both — and then Sendly sends email. That is the only channel it has.
So the comparison is genuinely simple. If your programme is email plus SMS plus push plus in-app, Sendly covers one quarter of it and you will still need the rest somewhere. If your programme is email, and the multi-channel platform is being paid for one channel, that is the situation Sendly is a straightforward answer to.
What lines up, and what does not
| Capability | In Sendly today |
|---|---|
| Track custom events on a contact | Yes. Events are a first-class object with an API, queryable, and usable as a workflow trigger and in segment rules. |
| Behavioural segments that re-evaluate | Yes. Dynamic segments are built from contact fields and tracked behaviour and stay current on their own. |
| Visual workflows with branching and waits | Yes, with a per-contact execution log so you can see which branch someone took and why. |
| Transactional sends from your application | Yes, over the same REST API and against the same contacts. |
| Broadcast campaigns to a segment | Yes, with scheduling, pause and resume, and per-campaign stats. |
| Inbound email on your own domain | Yes — real mailboxes over IMAP and SMTP, also projected into the API. |
| SMS, push, in-app messages, webhooks as a channel | No. Sendly is an email platform. |
| A data warehouse sync or reverse-ETL destination | No. |
| Liquid-style templating across channels | Templates with variable substitution, for email only. |
There is no compat endpoint for Customer.io. Sendly speaks the transactional-send dialect of five providers, and Customer.io is not one of them, so a move is an import and a rebuild rather than a base-URL swap. Contacts and suppression come across as data; journeys are recreated as workflows.
Where the money actually goes
Event-driven platforms are usually priced on the size of the audience they hold, sometimes with a separate line for the volume of events you send them. Sendly is priced on emails sent, and on nothing else.
That is not automatically cheaper, and the honest answer depends on a ratio you can work out in a minute: how many emails do you send per contact per month? A large, rarely-mailed audience is the case where per-email pricing wins by a lot. A small audience mailed daily is the case where it does not. Do that arithmetic before you do anything else.
Tracked events are not billed separately here, and neither are stored contacts. There is no ingestion charge for streaming behaviour in.
What Sendly does
Sendly is one workspace for transactional email, marketing campaigns and automation, against one contact database. Everything below is in the product today, not on a roadmap.
| Area | What you get |
|---|---|
| Transactional send | REST API with idempotent sends, batch send, scheduling and cancellation, template variables, attachments, and a delivery record you can read back. |
| Campaigns | One-off broadcasts to a list or a segment, with scheduling, pause and resume, and per-campaign stats. |
| Workflows | A visual builder for automated sequences, with branching on contact data and on tracked events. Runs server-side, with an execution log per contact. |
| Contacts and segments | Custom fields, CSV import, and dynamic segments that re-evaluate from contact data and behaviour. Built for millions of rows, with cursor pagination throughout. |
| Deliverability | Domain verification with guided DNS setup that publishes the records for you and then watches them for drift. SPF, DKIM and DMARC alignment, suppression list, bounce and complaint handling. |
| Inbound | Real mailboxes on your own domain, reachable over IMAP and SMTP, with the messages also projected into the API. |
| Events and webhooks | An event stream you can query and subscribe to, signed webhook deliveries with retries, and tracked custom events that can trigger a workflow. |
| AI agents | A remote MCP server, so an agent can work in your account under permissions you tick yourself. Irreversible ones are never granted by default. |
| SDKs | Official open-source clients for JavaScript and Python, both at 1.0.0, generated from the published OpenAPI contract. |
| Self-hosting | Documented Docker deployment, if you would rather run it yourself. |
What Sendly costs
There is no monthly platform fee and there is no plan to outgrow. A free tier, metered usage that gets cheaper as volume rises, and a conversation for anything past that.
| Plan | Price | What it is |
|---|---|---|
| Free | $0 / month · forever | Everything you need to get started. No credit card required. 5,000 emails / month · Transactional, campaigns & workflows · Full REST API, SDKs & webhooks · Powered-by-Sendly badge on sent email |
| Pay as you go | $1.00 / 1,000 emails | Volume pricing that drops as you send more. No monthly fee and no plan to outgrow. $1.00 per 1,000 emails up to 50,000 · $0.70 per 1,000 above 50,000 · $0.50 per 1,000 above 250,000 · Badge removed from your email · Set your own limit per category · Billed in USD, EUR or GBP |
| Custom | Let's talk tailored to your volume | High-volume or regulated sending that needs limits set by a human. Everything in Pay as you go · Custom sending limits · Higher daily throughput · Direct support from the team |
The free tier is a real tier, not a trial: it does not expire, and nothing about it is time-limited. Its allowance is shared across transactional sends, campaigns, workflows and inbound mail, because Sendly meters emails rather than products.
What to check on Customer.io's side
We do not publish Customer.io's prices or limits here. They change without telling us, and a stale number on this page would be the one thing on it you could catch us being wrong about. These are the questions worth answering from their own documentation before you move anything.
- What does the bill look like at your actual monthly volume, and at ten times it? Flat plans and metered curves cross over somewhere; find where.
- Are transactional sends and marketing sends billed and rate-limited from the same allowance, or two?
- Is there a contact-count charge on top of the send charge, and does an unsubscribed contact still count?
- What happens on the day you exceed the plan — a hard stop, an overage rate, or an upgrade prompt?
- Can you export your contacts, templates and suppression list in full, without asking support?
- Is there an API for everything the dashboard can do, and is the contract published?
Questions people ask
- Does Sendly support SMS or push?
- No. Sendly is an email platform: transactional sends, campaigns, workflows and inbound mail. If your messaging programme needs other channels, Sendly will not replace a multi-channel platform, and no amount of roadmap talk should be read into that.
- Can I trigger a workflow from an event in my product?
- Yes. Record a custom event against a contact through the API, and a workflow can use it as its trigger or branch on it further down. The same events are queryable and usable in segment rules.
- Is there a compat endpoint for Customer.io like there is for Resend?
- No. The compat endpoints cover the transactional-send path of Resend, SendGrid, Postmark, Mailgun and Plunk. Customer.io's API is a different shape and a different job, so moving means importing contacts and suppression and rebuilding journeys as workflows.
- Are tracked events billed?
- No. Sendly meters emails sent. Contacts stored and events recorded do not carry their own charge, so streaming behaviour in does not change the bill.
- How do journeys map onto workflows?
- Concept for concept, mostly: a trigger, a wait, a branch on contact data or a tracked event, a send. The builder is different and there is no import, so somebody rebuilds them by hand. Start with the two or three that carry real volume and leave the long tail until you have data saying it matters.
Something on this page out of date, or missing the case you are in? Write to support@sendly.now — we answer our own email. The full technical detail lives in the documentation.